© Fraunhofer IFAM

Integration of thermally active materials during the formation of open porous metal structures

Jörg Weise1, Joachim Baumeister1, Marc Möllers2, Hannes Seifarth2, Sebastian-Johannes Ernst2

1Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM, Bremen

2Fraunhofer Institute for Solar Energy Systems ISE, Freiburg

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System: Adsorption and desorption effects

allow to ”pump“ heat between different temperature levels

E.g. water adsorption by zeolite Essential components are the adsorber

and the evaporator:

Are basically heat exchangers, combined with thermally active material (zeolite)

Made from metal (Al, Cu, steel)

Crucial components, determine system efficiency and power density

Introduction

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Introduction

Combining heat exchangers with thermally active material, SotA:

Loose bulk of zeolite granules between the lamellae of the metal heat exchanger

Bad thermal contact zeolite/metal

Coating of the lamellae with mixture of zeolite powder/binder

Limited specific surface

Difficult coating of fine structures

How can adorber performance be improved?

[ISE]

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Introduction

Requirements influencing the performance of an adsorber:

High thermal conductivity

Heat transfer from active material to fluid system

Low (thermal) mass

”Dead weight“ will lower coefficient of performance

Large surface

Thinner coating improves system kinetics

Accessibility

For coating and for reactants during system operation

Good adhesion and thermal contact of active material

Long time stability

Costs

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Conventional approach (SotA)

Introduction

Metal Active material

Production of metal semi-finished parts (lamellae, fibres, pipes,…)

Combination with active material (filling, coating)

Assembly to metal adsorber structure (soldering,…)

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Alternative approach:

New technological approach

E.g.

Fabrication of the porous metal structure can be done by casting or sintering

Direct integration of the active material during the fabrication of the metal structure

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Production of composite structure - casting

Basis: casting of metal sponges

Organic place-holder structure

Pressure assisted metal melt infiltration

Pyrolysis of place holder

Adapted approach:

Integration of granules of the active material into the preform

“Granules transplantation” to solidifying metal during casting

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New approach (casting):

coating of the place holder net with zeolite granules

Pla

ce h

old

er

Production of composite structure - casting

© Fraunhofer

Act

ive m

ate

rial

Production of composite structure - casting

New approach (casting):

coating of the place holder net with zeolite granules

Pla

ce h

old

er

© Fraunhofer

Meta

l Production of composite structure - casting

New approach (casting):

coating of the place holder net with zeolite granules

infiltration with metal melt, “transplantation” of the zeolite granules to solidifying metal during casting

Act

ive m

ate

rial

Pla

ce h

old

er

© Fraunhofer

Production of composite structure - casting

New approach (casting):

coating of the place holder net with zeolite granules

infiltration with metal melt, “transplantation” of the zeolite granules to solidifying metal during casting

removal of the placeholder structure (e.g. by pyrolysis)

Meta

l

Act

ive m

ate

rial

© Fraunhofer

Production of composite structure - casting

Example: open cell aluminium sponges, zeolite granules integrated into the surface of every strut

Very good thermal contact between zeolite and heat exchanger

Ratio of zeolite/metal and porosity can be adjusted

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Production of composite structure - casting

Placeholder EVA polymer net

Coating with zeolite granules

Wrapping around fluid pipe

Final shaping of preform

Infiltration, machining, polymer pyrolysis,

assembly

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Production of composite structure - sintering

New approach (sintering):

Coating of the active material granules with binder

Mixing with metal powder/flakes/fibres, adherence to granules surface

Transfer to mould, shaping

Sintering

Finished porous composite

Cu-Fibres Cu-Powder Cu-Flakes

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Combination of metal foam with

+ expanded glas granules (boiling stones)

+ zeolite (adsorbens)

evaporator adsorber

Production of composite structures

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Expanded glass integration - evaporator

New composite evaporator structure:

expanded glass granules act as nucleation sites (bubble formation)

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Expanded glass integration - evaporator

Impact of expanded glass granules

High wall superheat: Sponge sample without granules performs better

Low wall superheat: Sponge sample with granules performs better relevant for adsorption heat pump application!

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combination metal foam +

+ expanded glas granules(boiling stones)

+ zeolite (adsorbens)

evaporator adsorber

Production of composite structures

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Zeolite integration - adsorber

Variable compositions possible

Typical: 50vol% zeolite, 10vol% binder, 15vol% metal, open porosity for vapour transport 20vol%

Size of zeolite granules: >60µm

Thermal conductivity (Cu, Al): 2-5 W/m/K

Active materials like zeolites are often not stable at high temperatures.

Casting/sintering: >600°C

Investigation of zeolite degradation

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Water adsorption behaviour of zeolite is retained

Structure is affected by chosen process slight degradation of sample IFAMB-18

XRD investigation shows a slight shift of peaks and some amorphous background only on sample IFAMB-18

No signs of degradation for IFAMB-19 and IFAMB-20

Zeolite integration – adsorber zeolite – stability

0.0 0.2 0.4 0.6 0.8 1.0

0

50

100

150

200

250

300

350

wat

er u

pta

ke [

mg

/ g]

rel. pressure [p / p0]

KOE-14 / Pure zeolite

IFAMB-18 / glass granules + zeolite

IFAMB-19 / EVA-precurser + zeolite

IFAMB-20 / zink + zeolite

water adsorption @ 25°C

5 10 15 20 25 30 35 40 45 50

2q / °

IFAMB-20

IFAMB-19

IFAMB-18

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Successful manufacturing of an adsorber module made of zeolite-Al-composite

Directly integrated fluid pipes

Successful measurements on manufactured adsorber give promising results

Zeolite integration – adsorber zeolite – potential

0 10 20 30 40 50

0

200

400

600

800

1000

po

we

r [W

]

time [min]

Ads:35°C - Evap:10°C

Ads:49°C - Evap:4°C

0 10 20 30 40 50

0

20

40

60

wat

er u

pta

ke [

g]

time [min]

Ads:35°C - Evap:10°C

Ads:49°C - Evap:4°C

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Summary and conclusion

New approach for production of porous metal structures combined with active materials

Integration of the active material in-situ during the formation of the metal porous structure

Simple process

Ratio of active material/metal and porosity can be adjusted

Application: evaporator and adsorber for adsorption heat pumps / chillers

Evaporator: significantly improved boiling behaviour

Adsorber: using suitable processing parameters the degradation of zeolite can be avoided

The work was done in the framework of the project „Harvest“ funded by the Fraunhofer-Zukunftsstiftung.